Why are these tiles appearing in my final rendered image?
Figured out the cause thanks to @Rich Sedman, so I guess this is a temporary "solution", as it doesn't really solve the problem, just finds a way around it.

The tiles appear in the render due to CPU and GPU handling the output differently for some reason. Deactivating the CPU in the CUDA settings not only solved the issue but also gave me 30 seconds faster render times per frame.

Not sure if that's an issue of the current 2.8 release candidate or just something that comes with combining two pieces of hardware for the same process. (Maybe my Geforce GTX 1080 ti doesn't work well with my AMD Ryzen 7 2700X?)